Ostertagia ostertagi is the most economically important helminth parasite of cattle in temperate parts of the world(1). As with all gastrointestinal nematodiases of ruminants, ostertagiasis is controlled almost entirely by the use of anthelmintic drugs combined with pasture management. Unfortunately, reports of anthelmintic resistance in bovine gastrointestinal nematodes are becoming more frequent. For example, benzimidazole (BZ) resistance was identified in cattle in New Zealand(2) and South America (3), and macrocyclic lactone (ML) resistance in New Zealand (4,5), the Americas (6,7,8) and Europe (9,10,11,12), However, as no detailed surveys have been made, the extent of the problem remains unknown and probably underestimated. It is anticipated that anthelmintic resistance in cattle nematodes including Ostertagia is likely to follow the pattern experienced with sheep, where it has reached serious proportions (13,14,15,16) although the pace at which it develops and spreads is likely to be slower.
Alternative methods for controlling bovine ostertagiasis remain an attractive prospect, in part due to the threat of drug resistance and partly because of increasing consumer sensitivity to the possibility of chemical residues in meat and milk. One such possibility for control is by vaccination. Earlier attempts to do this, using either infection with irradiated larvae(17,18) or immunisation with crude somatic or excretory/secretory products of the parasites(19,20) were not successful. More recently promising results have been obtained using fractionated native excretory/secretory products of adult parasites, with reductions in faecal egg counts of up to 80%(21,22,23,24,25).
During the last 15 years or so substantial protection against the important blood sucking ovine nematode, Haemonchus contortus, has been achieved by immunising sheep with various antigens isolated from the intestinal membranes of adult parasites, a topic which has been reviewed extensively(26,27,28). When the same gut antigen approach was tested against Ostertagia, some protection was conferred, but not at a level deemed to have practical potential(29). However as the same O. ostertagi antigens cross-protected efficiently against Haemonchus in sheep, it was reasoned that the relative failure may have been because adult Ostertagia are not blood feeders and do not ingest sufficient antibody for the gut antigen approach to be highly effective against them.
It was hypothesised that developing fourth stage (L4) Ostertagia might be more vulnerable to this type of vaccination than adult parasites. Firstly, since they inhabit and damage the gastric glands, L4s are likely to be continuously exposed to inflammatory exudate which is richer in host immunoglobulin than the mucous surface inhabited by adult worms, in other words, L4s in vaccinated calves would be expected to ingest a bigger dose of antibody per unit weight than adult worms. Second, because developing Ostertagia L4s grow very rapidly (30), increasing their mass about 20-fold in 10 days, they may be more sensitive to digestive interference than their slower metabolizing adult counterparts.